A display device is comprised of an array of pixels. “Electro-optic” refers to an effect in which the optical properties of a material change in response to an electric field. Optical properties may include, but are not limited, to visual appearance. A display device in which the optical properties of the pixels may be made to change in response to an electric field is an electro-optic display device. Generally, if a pixel has two stable states, it may be considered bistable. However, the term “bistable” is used herein to mean multi-stable, i.e., pixels may have two or more stable states. For an optical property of a pixel to be considered stable, it is required that the property remain stable for a non-temporary period of time. Such a time period may be considered non-temporary if it is four times the minimum duration of a driving signal required to produce the electric field or fields needed to change the optical property of a pixel.
A variety of bistable, electro-optic display devices are known. One type of bistable, electro-optic display employs rotating bichromal bodies. A large number of small bodies having two or more sections with differing optical characteristics and an internal dipole are suspended in minute, liquid-filled cavities of a matrix. The bodies may be made to rotate by applying an electric field. Another type of bistable, electro-optic display uses an electrochromic medium. Yet another type of electro-optic display employs electro-wetting.
One type bistable, electro-optic display device is based on electrophoresis, that is, the movement of charged particles in response to an electric field. In displays of this type, charged particles are made to move through a fluid by application of an electric field. The fluid may be liquid or gas. The particles may be enclosed in a capsule, and the particles, fluid, capsules, and other elements may be collectively referred to as “encapsulated electrophoretic media.” Typically, encapsulated electrophoretic media comprise numerous small capsules, and the capsules are held within a polymeric binder to form a coherent layer positioned between two electrodes. Displays of this type may be referred to as microencapsulated electrophoretic (“MEP”) displays or electrophoretic displays (“EPDs”).
A “micro-cell” electrophoretic display is another type of EPD. In a micro-cell electrophoretic display, the charged particles and the suspending fluid are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film.
EPDs may be made so that they can be updated using a thin-film transistor (“TFT”) array similar to those employed in active matrix LCDs, in which case the EPD may be referred to as an active matrix EPD (“AMPED”). EPDs are a promising technology for use in electronic readers or “e-books” because they provide good readability in a variety of lighting conditions, including sunlight. In addition, they consume low amounts of power compared with LCDs.
EPDs have been designed to operate using several different update modes. A “monochrome” update mode may be used to refresh an image comprised entirely of black and white pixels, such as text. An image of this type may also be referred to as a bi-level or bit-mapped image. A gray-scale update mode is used to refresh an image comprised of pixels that may be black, white, or a shade of gray in between black and white.
The time required to refresh an EPD in different update modes varies. For example, one exemplary monochrome update mode may require about one-third of the time an exemplary gray-scale update mode may require to update the EPD. It is generally desirable to use the fastest available update mode. However, the use of an incorrect update mode may result in image artifacts.
The image displayed on an EPD may only need to be refreshed every ten minutes or so, or when the image changes. For this reason, a CPU or other image data source generally needs to specifically request an image update. When the CPU requests an image update, it needs to specify an update mode and additional parameters.
There is a technical problem with known EPD s. Specifically, a CPU or other source of image data must specify an update mode and additional parameters. However, the information needed to determine the required update mode and other parameters may not be readily available to the CPU. Further, the determination of parameter values may place a significant burden on the CPU.
Accordingly, there is a need for methods and apparatus that simplify the interface between a CPU or other source of image data and a bistable, electro-optic display device, such as an EPD.